1. Field of the Invention
The present invention relates generally to a photodiode integrated circuit having multiple gain states and, more particularly, to a photodiode integrated circuit having multiple gain states, which is capable of achieving multiple gain states through a single output stage circuit using a multiplexer. Furthermore, the present invention relates to a photodiode integrated circuit having multiple gain states, which is capable of stabilizing the input offset voltage of an amplification unit using a current-voltage conversion circuit having multiple gain states.
2. Description of the Related Art
Recently, the trend of the optical storage market is rapidly changing from Compact Disk-Read Only Memory (CD-ROM) devices to Digital Video Disk (DVD) recording devices.
With such a trend, optical storage drive manufacturers are developing various technologies for high-speed recording on a DVD. The drive manufacturers research various servo schemes and apply them to actual products to develop high-speed DVD drives.
Most drive manufacturers employed a sample and hold scheme before the advent of 4× DVD drives. However, since 8× DVD drives were introduced, the trend is changing from the sample and hold scheme to another servo scheme, an average servo scheme.
The average servo scheme determines whether a signal level is 0 or 1 by averaging output pulses reflected from a disk and comparing the average value with a reference voltage. For this reason, in the case of the average servo scheme, various output levels are required. Accordingly, a PhotoDiode Integrated Circuit (PDIC) requires various gain states. Furthermore, there are even cases in which ten or more gain states are required.
In the case in which the drive manufacturers employ the average servo scheme, the PDIC requires multiple gain states. That is, ten gain states are required for an 8× optical storage device, and sixteen gain states are required for a 16× optical storage device.
FIG. 1 is a circuit diagram showing a conventional PDIC having two gain states, that is, a high gain state and a low gain state. Generally, in the case of a PDIC for a recording device, the two gain states are a low gain state for a write operation and a high gain state for a read operation. An output level depending on the power of a laser diode, that is, the light-emitting element of the PDIC, is maintained constant by causing the gain at the time of the write operation to differ from the gain at the time of the read operation.
The PDIC includes a current-voltage conversion unit 11 for converting current, which is generated by an optical signal, into voltage, and an amplification unit 12 for amplifying a voltage signal at a gain corresponding to an operational state and then outputting the amplified voltage signal.
The current-voltage conversion unit 11 includes a photodiode PD, that is, a photoelectric conversion element, an amplifier 13, a feedback resistor RIV and a reference resistor RREF1.
The amplification unit 12 includes an input amplifier 14, that is, an input stage circuit, a high gain output amplifier 15, a high gain feedback resistor RHG, a first switch SW1, a low gain output amplifier 16, a low gain feedback resistor RLG, a second switch SW2, and a reference resistor RREF2. The input amplifier 14 has a predetermined gain dependent on the ratio of the resistance of the reference resistor RREF2 to the resistance of the feedback resistor RHG or RLG, and the output amplifiers 15 and 16 have a gain of 1.
At the time of a high gain operation, the first switch SW1 is turned off on and the second switch SW2 is turned off, so that the output voltage is as follows:
      V    OUT    =            [              1        +                              R            HG                                R                          REF              ⁢                                                          ⁢              2                                          ]        ·          V      IN      where VIN is a voltage value that is applied by the current-voltage conversion unit 11 to the non-inverting terminal of the input stage circuit 14 of the amplification unit 12.
Similarly, at the time of a low gain operation, the first switch SW1 is turned off and the second switch SW2 is turned on, so that the output voltage is as follows:
      V    OUT    =            [              1        +                              R            LG                                R                          REF              ⁢                                                          ⁢              2                                          ]        ·          V      IN      
Since, at the time of the write operation, a high input voltage based on a relatively strong optical signal is transferred, the input voltage VIN is amplified and output in a low gain state. Since, at the time of the read operation, a low input voltage based on a relatively weak optical signal is transferred, the input voltage VIN is amplified and output in a high gain state.
Many conventional optical storage devices include separate output stage circuits 15 and 16 for required operational states, for example, the buffer 15 and feedback resistor RHG for the high gain state, and the buffer 16 and feedback resistor RLG for the low gain state, as shown in FIG. 1, and perform gain switching.
However, the case of FIG. 1 is disadvantageous in that it is difficult to set an offset voltage that satisfies both the high and low operational states.
That is, the feedback resistors have different resistances in the high and low gain states, and a feedback current passing through the feedback resistor RHG in the high gain state and a feedback current passing through the feedback resistor RLG in the low gain state also have different values due to the different feedback resistances. Accordingly, the voltage applied to the reference resistor RREF2 of the amplification unit 12 also varies, so that an offset voltage applied to the input terminal of the input amplifier 14 varies according to the gain state. If the offset voltage that varies according to the gain state is not precisely set, variation in the output level occurs, which causes errors.
Furthermore, a problem arises in that cross talk occurs on a circuit because two output stage circuits are used for two operational states. The cross talk in the PDIC refers to the following case.
For example, the high gain output stage circuit 15 must not operate in a low gain operational state. However, in the conventional circuit shown in FIG. 1, the high gain output stage circuit 15 is not completely turned off in the low gain operational state, and outputs a specific signal. A similar phenomenon occurs in a high gain operational state. Such a cross talk phenomenon affects an output voltage level, thus causing errors in the output voltage level.
Although only the case in which the PDIC has two gain states, that is, a high gain state and a low gain state, has been described above as an example with reference to FIG. 1, the above-described problems may be still more serious in the case in which three or more operational states exist, that is, the case in which eight or sixteen gain states exist.
In relation to the prior art, Japanese Unexamined Pat. Pub. No. 2000-138548 discloses a variable gain inverting amplification circuit in which a multiplexer is mounted on a feedback capacitor. However, since the variable gain inverting amplification circuit has a simple construction capable of varying only the gain of the feedback capacitor, it cannot be more effectively used for the PDIC.
For PDICS, a circuit that is capable not only of providing a variable gain using a single amplification stage circuit and, but also of varying an offset voltage on an input terminal side according to application, is required.